For me, the only thing Mark IV has over V is CH3, and a mountable foot controller.
Everything else, I find V is just superior.
I sold my IVa few years ago kidding myself I was going to be a blues guy.
I bought a V about a year ago and have been really happy with it
CH1 has more sprakling clean with more tonal pallette with 3 modes.
CH2 crunch mode has more gain and can get both amazing rhythm and lead tone.
Lead boost, tube recifier option, more wattage option, etc.
Mark IV CH3 is a killer lead channel though.
It just has that raw, aggressive tone which sounds wicked for metal more so than V's IV mode. I'm not saying V can't deliver amazing high gain tone, but IV CH3 just does a more satisfying job for me. Still I had few chances to get MarkIV back for a great price last year, but I just passed on them.
For me I love them both but lean towards V more for its flexibility.